Apparatuses for testing multi-core cables for leakage between cores

ABSTRACT

In order to test the leakage between cores of a multicore cable, a test voltage is applied by a first switch in turn to the cores of a cable whose other end is disconnected. A second switch selects the remaining cores in turn and, after each new selection is made by the second switch, the selected core is momentarily connected to a predetermined voltage, e.g., ground or earth potential, by a third switch. This allows the capacitor formed by the two cores to be rapidly charged so that current measuring devices for detecting leakage current do not give spurious readings caused by the charging current. Also, all pair-combinations of the cores may be checked rapidly, allowing cables with large numbers of cores to be tested efficiently.

The present invention relates to apparatuses for testing cables. Such anapparatus is particularly useful for the semi-automatic testing ofmulticore cables.

According to one aspect of the invention, there is provided an apparatusfor testing for leakage between cores of a multicore cable, broadlycomprising first means for connection to one end of the cable while theother end of the latter remains disconnected, second means for applyinga test voltage through the first means to a first cable core for apredetermined period, third means for connecting and setting eachremaining cable core in turn through the first means to a predeterminedvoltage during an initial portion of the predetermined period and fordisconnecting each of the said remaining cable cores in turn from thepredetermined voltage at the end of the said initial portion of thepredetermined period, and fourth means for detecting current in each ofthe said remaining cores.

Preferably the first means for applying a test voltage comprises a testvoltage source and first switch means for connecting in turn each of atleast some of the cable cores to the voltage source through a suitableconnector constituting the first means. Preferably the fourth ordetecting means comprises an ammeter or a voltmeter and second switchmeans for connecting in turn each of at least some of the cable cores tothe ammeter or voltmeter.

The switch means preferably comprise electrically or electronicallyoperable switches, for instance electromagnetic relays, driven by acounter so as to provide automatic switching of the cores. Preferablymeans are provided for storing and/or displaying an indication of anycores between which leakage is detected.

Preferably the third setting means comprises third switch means forconnecting momentarily the second core to a reference potential, forinstance earth. Preferably the third switch means is arranged to beactuated immediately after each actuation of the first or second switchmeans. The third switch means may comprise an electromagnetic relaywhose contact is in parallel with a semiconductor switch, for instance atransistor of an optocoupler or opto-isolator.

When testing for leakage or insulation faults in multicore cables, everycombination of pairs of cores must be tested. For a cable comprising Ncores, the number of combinations of pairs is (N-1)+(N-2)+ . . .+3+2+1=(N² +N)/2-N, and manual checking is tedious, laborious, andtime-consuming. For a cable of substantial lenght, there is substantialand practically unpredictable capacitance between each pair of coresand, unless care is taken, the effects of this capacitance can upsettesting and give rise to spurious and incorrect results.

The apparatus defined above makes it possible to provide automatic orsemi-automatic testing of a cable rapidly and reliably. By charging eachcore fully prior to testing, the cable effects mentioned above areovercome and speed of operation is limited essentially by the periodduring which the setting means must be allowed to operate in order topermit the core to be reliably charged. The use of a semiconductorswitch allows the charging means to begin charging the core veryrapidly, and the provision of a parallel relay contact allows chargingto be completed beyond the point where, for instance, the finiteconduction cut-off collector-emitter voltage would prevent thesemiconductor switch from totally charging the core.

According to another aspect of the invention, there is provided anapparatus for testing a multicore cable, comprising a plurality of testresistors of different values and each having a first terminal connectedto a common line and a second terminal arranged to be connected to arespective one of at least some of the cable cores at one of the cable,and means for determining whether the resistance between the common lineand each of the at least some cores at the other end of the cable iswithin a predetermined range corresponding to the respective testresistor to which the core is connected.

Preferably the common line is one of the cable cores.

Preferably the determining means comprises a voltage source arranged tobe connected to the common line, an operational amplifier, first switchmeans for connecting in turn each of the at least some cores at theother end of the cable to the inverting input of the operationalamplifier, adjustable negative feedback resistance means, second switchmeans for adjusting the negative feedback resistance means to a valuewhich is a substantially constant ratio to the value of each of the testresistors, and means for determining whether the output voltage of theoperational amplifier is equal to a predetermined voltage within apredetermined range. Preferably the constant ratio is 1:2. Preferablyeach pair of adjacent value test resistors have resistances whose ratiois constant, for instance substantially equal to 1:1.2. Preferably thepredetermined range is ±20%.

It is thus possible to provide an apparatus for testing the integrity ofcore connections of a multicore cable. In particular, the apparatuschecks whether there is a short circuit between cores or whether theorder of connection of cores at one end of the cable differs from thatat the other end.

The invention will be further described, by way of example, withreference to the accompanying drawing, which is a schematic circuitdiagram of an apparatus constituting a preferred embodiment of theinvention.

For the purpose of simplicity and clarity, the apparatus shown in theaccompanying drawing is capable of testing multicore cables having fivecores. However, in general, practical embodiments of the apparatus willbe capable of testing cables having a much larger number of cores, forinstance up to twenty cores, up to one hundred cores, or up to anypractical number of cores.

The apparatus comprises a voltmeter 1 which is illustrated as having aconventional moving coil analogue meter display 2 but which couldinclude any suitable display. The voltmeter has a first input connectedto a switch 3 for selectively connecting the first input to the outputof an operational amplifier 4 or to a switch 3' with which the switch 3is ganged. The switches 3 and 3' thus function as ganged change-overswitches and are provided by one or more electronic switches,electromagnetic relays, or any suitable switching means.

The voltmeter 1 has a second input connected to a switch 6. Thevoltmeter also has an output connected to a comparator 7.

The operational amplifier 4 is provided with a feedback diode 8 whichsubstantially prevents positive voltages appearing at the output of theamplifier. The amplifier is also provided with two different feedbacknetworks selected by a pair of change-over switches 9 and 10 which areoperated electrically or electronically and are ganged together. In afirst position of the switches 9 and 10, a single feedback resistor 11,for instance having a value of 1 Megohm, is switched into circuit. Inthe other position of the switches 9 and 10, a network comprising anelectrically or electronically actuated switch 12, resistors R1, R2, R3,and R4, a switch 13, and resistors 14, 15, and 16 is selected as thenegative feedback loop of the operational amplifier 4.

The inverting input of the amplifier 4 is connected via the switch 3' tothe output of a switch 5 which comprises a plurality of electromagneticrelays and contacts 5a-5e. Each relay contact is connectable between theinput of the amplifier 4 via the switch 3' and a respective pole of aninput connector 17 into which a multicore cable to be tested may beplugged directly or via an adaptor.

The input of the amplifier 4 is also connected to earth by the parallelcombination of a first series circuit comprising a switch 18 constitutedby a relay and a current-limiting resistor 30, and a second seriescircuit comprising the collector-emitter path of a transistor formingpart of an optocoupler or opto-isolator 19, a switch 31, and acurrent-limiting resistor 32.

The poles of the connector 17 are also connected to a switch 20 which isof the same type as the switch 5 and comprises relays and relay contacts20a-20e. The switch 20 is connectable by means of the switch 6 either toa voltage source +V or to the second input of the voltmeter 1.

The output of the comparator 7 is connected to a logic circuit 21 whichcontrols operation of the apparatus by means of a decoder driver circuit22 which has an output bus 23 for controlling voltmeter 1, thecomparator 7, the switches, and the optocoupler 19 and switch 18. Thelogic circuit 21 also drives two digital seven segment liquid crystaldisplays 24 and 25. In addition, the logic circuit 21 includes akeyboard 33 for permitting a user to actuate the apparatus so as toperform various tests on the multicore cable.

The elements of the apparatus thus far described are provided in asingle housing which, in use, is located at one end of the cable to betested. In addition, the apparatus comprises two further separate partsfor connection to the remote end of the cable in order to performcorresponding tests. The first of these parts comprises a connector 26into which the remote end of the cable may be plugged either directly orby an adaptor. The poles of the connector 26 are all connected together.

The second part also comprises a connector 27 into which the remote endof the cable may be plugged directly or via an adaptor. One of the polesof the connector 27 is connected by a common line 28 to first terminalsof resistors R5, R6, R7, and R8. The other terminals of these resistorsare connected to respective poles of the connector 27. A switch 29allows the common line to be connected to earth.

Operation of the apparatus to test a cable is as follows. The cable isplugged into the connector 17 either directly or via an adaptor and theapparatus is switched on. The number of cable cores is manuallyprogrammed into the logic circuit 21 by the keyboard 33. A check is thenperformed on the cores of the cable to ensure that no voltages arepresent, as might be the case if the remote end of the cable had notbeen disconnected for the purpose of testing. Initially, the switch 3connects the first input of the voltmeter via the switch 3', whichisolates the input of the amplifier 4, to the output of the switch 5,the switches 9 and 10 select the resistor 11, and the switches 18 and 20and the optocoupler 19 are switched off. The logic circuit 21 contains acounter 21' which is reset at switch-on and then counts up to theprogrammed number of cores. The decoder driver 22 decodes this count andactuates the corresponding one of the relays forming the switch 5 sothat the cores of the cable are connected in turn to the input of thevoltmeter 1 by the contacts 5a-5e. The decoder driver 22 controls viaits output 23 the sensitivity of the voltmeter 1 so that the outputsignal supplied to the comparator 7 falls within a predetermined usablerange of the comparator with respect to a reference voltage VR suppliedthereto.

During the initial test, the apparatus checks to see whether anypotentially harmful voltage is present between each of the cores andearth and, if any voltage is detected which exceeds a preset valuecorresponding to the reference voltage VR, the comparator 7 supplies anoutput signal to the logic circuit 21, which disengages all relaycontacts 5a-5e forming the switch 5 so as to prevent damage to theapparatus and provides a warning indication, for instance by means ofone or both of the displays 24 and 25, one of which may indicate thecore on which the voltage is present. The apparatus remains in this modeuntil reset by the user after having rectified the potentially harmfulsituation, in particular by ensuring that both ends of the cable aredisconnected from any other apparatus, and the cable is not in contactwith another separate cable, for instance in a common cable tray.

If this initial test is satisfactory, then the apparatus checks whetherany potentially harmful voltages are present between any pair of thecores of the cable. The switch 6 connects the switch 20 to the secondinput of the voltmeter so that the voltmeter 1 measures the differencebetween the voltages on its first and second inputs. The switch 20aconnected to a first cable core is closed under control of the logiccircuit 21 and the decoder-driver 22, and the switch contacts 5b-5eselect in turn each of the other cable cores. The switch contact 20athen opens and the contact 20b closes to select a second cable core andthe switch contacts 5c-5e select in turn the remaining cores. Thissequencing by the switches 5 and 20 continues until every combination ofpairs of the cable cores has been selected, provided no potentiallyharmful voltage has been detected. If such a harmful voltage isdetected, then the relays forming both switches 5 and 20 are disabled soas to prevent damage and an indication is provided to warn the user. Forinstance, the displays 24 and 25 may indicate the cores between whichthe voltage has been found. The apparatus remains in this conditionuntil manually reset, again following remedial action to remove thepotentially harmful core-to-core voltage.

Following detection of a potentially harmful core-to-earth orcore-to-core voltage, resetting of the apparatus may be effective eitherto start the testing sequence from the beginning or to recheck thecombination which gave rise to the harmful condition and then proceedwith the remainder of the test sequence. In the first case, the counteror counters within the logic circuit 21 are reset to zero whereas in thesecond case the counter or counters merely hold their count and areprevented from further counting until manually reset.

Once these two tests have been successfully completed, the apparatusrepeats these tests but with the sensitivity of the voltmeter 1increased so as to detect whether any non-harmful voltage is presentbetween any core and ground or between any pair of cores. If thehigh-voltage tests were satisfactory, then the further testing for lowvoltages is necessary so as to prevent erroneous results from theapparatus. During these two lower voltage test procedures, if a voltageis detected, then the sequence is stopped but the relays forming theswitches 5 and 20 remain operative so that the value of the voltage maybe read from the voltmeter display 2. The user is required to rectifythe condition and the test sequence then proceeds either manually orautomatically once the voltage has been removed or upon manual resettingof the apparatus. As soon as the lower voltage tests have beencompleted, the logic circuit 21 causes the apparatus to enter a waitingmode until one of the cable testing modes is manually selected by meansof the keyboard 33.

The apparatus provides three testing modes, namely for continuity of thecores, for leakage or faulty insulation between cores or between a coreand earth, and for short circuits between cores or incorrect connectionof the cores to connectors. Each of these three modes will now bedescribed in more detail.

When the core continuity mode is selected, it is necessary for theconnector 26 to be connected to the remote end of the cable. When thishas been done, the logic circuit 21 and decoder-driver 22 control theswitches 6 and 20 so that the voltage +V is supplied to one of thecores. For instance, the contact 20a is closed and the contacts 20b-20eremain open. The switches 3 and 3' connect the first input of thevoltmeter 1 to the switch 5. The logic circuit 21 and decoder-driver 22then cause the switch 5 to connect in turn each of the other cable coresby sequentially closing and opening the contacts 5b-5e, the contact 5aremaining open, and the voltage on each core is measured by thevoltmeter 1. The voltmeter 1 supplies a suitably scaled output signal tothe comparator 7 so that, in the presence of the correct voltage, thevoltage supplied to the comparator 7 exceeds the reference voltage VRand the comparator 7 supplies a signal to the logic circuit 21indicating that the core under test has an acceptable continuity. In theevent of a break in the core or the presence of a relatively highresistance because of a damaged or faulty connection or core conductorsuch that the voltage supplied to the comparator 7 is less than thereference voltage VR and the comparator 7 is not actuated, the logiccircuit halts the sequencing by the switch 5 and indicates on thedisplay 25 which core is faulty. Should there be a fault conditionduring the first test step, then there is a minor ambiguity in thateither the core connected to the switch contact 20a or the coreconnected to the switch contact 5b may be faulty. In this case, thedisplay 24 is also actuated. The user may then reset the apparatusmanually to continue checking the remainder of the cores for continuity.Alternatively, the apparatus may check each of these cores incombination with another to try to resolve the ambiguity.

In order to perform the leakage and insulation test, the remote end ofthe cable is disconnected manually so as to prepare for the test. Theleakage and insulation test mode is then manually selected and the logiccircuit 21 controls the switch 6 so that the test voltage +V is suppliedto the switch 20. The switch 3 connects the voltmeter 1 to the output ofthe amplifier 4, the switch 3' connects the switch 5 to the input of theamplifier 4, the switch 31 is closed, and the switches 9 and 10 selectthe resistor 11. The switch 20 then supplies the test voltage to one ofthe cores of the cable via the connector 17, for instance by closing thecontact 20a while the contacts 20b-20e remain open, and the logiccircuit 21 controls the switch 5 so that it cycles through the remainingcores, for instance by closing then opening the contacts 5b-5e in turn.Thus the switch contact 20a is closed to select a first core and theswitch contact 5b is initially closed to select a second core. Duringselection by the switch 5 and/or 20, the optocoupler 19 and the switch18 are actuated by the decoder driver so as to connect momentarily theselected core to earth. This allows the capacitance between the cores tobe charged rapidly via a low impedance path, so that the core connectedto the switch 5 is set to a predetermined, for instance earth,potential. Conduction of the transistor of the optocoupler 19 occursalmost instantaneously so that the level of charge is reduced until thevoltage between the core and earth falls to the collector-emitterCut-off voltage at which the transistor stops conducting. It takeslonger for the switch 18 to close but, when it does, it ensures that thecapacitance is fully charged. The combination of the optocoupler 19 andthe switch 18 substantially increases the speed with which the core canbe charged.

The output voltage of the amplifier 4, which is effectively functioningas a current-to-voltage converter or as a very high gain voltageamplifier, provides an indication of any leakage or inadequateinsulation between the cores selected by the switches 5 and 20. Thevoltmeter 1 supplies a corresponding voltage signal to the comparator 7and, if there is any substantial leakage, the logic circuit 21 issignalled by the comparator 7 and halts sequencing by the switches 5 and20. The cores being tested are indicated by the displays 24 and 25, andthe degree of leakage is indicated by the voltmeter display 2. Theapparatus may then be reset manually so as to continue sequencingthrough the cores. Thus, the switch contact 5b opens and the contact 5ccloses to select core three and the optocoupler 19 and switch 18 aremomentarily actuated. The switch 5 then selects cores four and five inturn so as to complete its sequence with respect to core one. The switchcontact 20a then opens and the contact 20b closes to select core two andthe switch contacts 5c-5e are closed in turn to select cores three tofive in sequence. Each time the switch 5 selects a different core, theoptocoupler 19 and the switch 18 are momentarily actuated. As soon asall combinations of pairs of cores have been tested, the apparatusreturns to a waiting mode.

As an alternative to the sequencing of cores during the leakage testdescribed above, the following sequencing may be performed and hasadvantages where a very large number of cores are being tested. Thecontact 20a is closed to supply the voltage +V to core one and thecontacts 5b-5e are simultaneously closed so that any leakage betweencore one and any of the other cores is detected. If an unacceptableamount of leakage is detected, the contacts 5b-5e are then opened andclosed in turn to determine the pair of cores between which leakage istaking place. Otherwise the contact 20a is opened, the contact 20b isclosed, and the contacts 5c-5e (or possibly the contacts 5a and 5c-5e)are simultaneously closed. This procedure is then repeated until theleakage test has been completed.

By testing for leakage between each core and all other cores (orremaining cores) simultaneously, the number of tests can besubstantially reduced, compared with testing between every combinationof core pairs, for cables with no, or only very few, leakage faults.Since it is relatively rare for there to be many leakage faults in acable, the time taken by leakage testing is substantially reduced.

The switching sequences followed by the switches 5 and 20 may bereversed. For instance, in the case where each core is tested forleakage to all other cores the sequence may start with the contact 5aclosed and the contacts 20b-20e closed so that the voltage +V issupplied simultaneously to cores two to five and leakage to core one istested.

A high voltage leakage test may be performed, for instance where +V =500volts, provided the electronic circuitry is properly protected. For thispurpose, the switch 31 is opened so as to protect the opto-isolator 19and a high voltage sensing circuit (not shown) is incorporated within orin place of the circuit comprising the op-amp 4 and associatedcomponents and the voltmeter 1.

In order to select the sequence mode, the remote end of the cable isplugged into the connector 27 either directly or via an adaptor. Thelogic circuit 21 then disables the switches 18 and 31 and theoptocoupler 19, and causes the switches 9 and 10 to select the feedbacknetwork comprising the switches 12 and 13, the resistors R1 to R4, andthe resistors 14 to 16. The voltmeter 1 remains connected via the switch3 to the output of the amplifier 4 whose input remains connected via theswitch 3' to the switch 5.

The values of the resistors R5 to R8 are selected such that the ratio ofR5 to R6 is equal to the ratio of R6 to R7 and to the ratio of R7 to R8.This ratio may, for instance, be 1:1.2. In practice, strict equality tothis ratio is not essential, and the actual nominal value of this ratiomay also be different.

Each of the resistors R1 to R4 has a value substantially equal to acommon fraction of the value of the corresponding resistor R5 to R8,respectively, i.e. R5/R1 =R6/R2 =R7/R3 =R8/R4. This fraction ispreferably a half, but other fractions would also be possible. In aspecific embodiment employing forty test resistors and forty feedbackresistors, the particular values used were as set out in the appendedtable.

The switch 13 and the resistors 14 to 16 are provided so as to equalisethe line resistance of the cores where this is significant in comparisonwith the smaller-value test resistors connected to the connector 27. Thevalue of the resistor selected by the switch 13 may be determinedmanually by measuring the resistance of one of the cores and selectingone of the resistors 14 to 16 having a value approximately equal to halfof this in the case where each feedback resistor is equal to half of thevalue of the corresponding test resistor.

In the preferred embodiment, the voltmeter display 2 is provided with 50evenly spaced calibrations. When the sequence test has been selected,the test voltage +V is supplied via the switch 6, the switch contact 20aand the first cable core to the common line 28. The switch contacts5b-5e then select sequentially the other cores and the switch 12sychronously connects the corresponding resistors R1 to R4 in thefeedback loop of the amplifier 4. In the absence of a fault condition,the voltmeter display 2 provides a reading which is within plus andminus 10 calibration points of half scale to indicate absence of afault. However, if a reading outside this range is obtained from thedisplay 2, this is indicative of a fault, for instance two cores shortedtogether or crossed-over cores. If full automation of the sequence testis required, the logic circuit 21 and the decoder-driver 22 cause thecomparator to function as a window comparator which, if it receives asignal outside the above range, supplies a signal to the logic circuit21 which suspends sequencing of the switch contacts 5b-5e until manualresetting allows sequencing to continue.

A modified form of the sequence mode may be used, for instance where thecable cores may have been crossed between the cable ends or where thecore which is connected to the common line 28 cannot be visuallyidentified at the other end of the cable. Initially, the connector 27 isconnected to the remote cable end and the switch 29 is closed so as toearth the common line 28. The contacts 20a-20e and the contacts 5a-5e,respectively, are closed in turn to determine the resistance throughcores one to five, respectively, to ground. The core connected to thecommon line 28 exhibits a resistance which is equal to the lineresistance plus earth resistance and which is the lowest of all measuredresistances. Both ends of the core may therefore be identified and, forthe succeeding core identification test, the voltage +V is supplied tothis core by the corresponding one of the contacts 20a-20e. The switch29 is opened to disconnect the common line 28 from earth and the switch13 is operated, manually or automatically, to select one of theresistors 14-16 to compensate for the line resistance i.e. theresistance of each core.

In order to identify the cores, the switch 12 first selects the resistorR1. The switch contacts 5a-5e (possibly excluding that connected to thecore to which the voltage +V is supplied) are closed in sequence untilthe comparator 7 signals to the logic circuit 21 that the measuredvoltage is within the predetermined window. The display 24 indicates thecore end at the apparatus which corresponds to the core end at theconnector 27 connected to the resistor R5. The logic circuit then awaitsa manually entered command to continue from the keyboard 33 beforeidentifying the next core end. Alternatively, if means are provided forautomatically storing the identity of the core, the logic circuit thenautomatically identifies the next core.

The next core is identified by causing the switch 12 to select theresistor R2 and by sequentially closing those of the contacts 5a-5ewhich are connected to cores which have not yet been identified. Thisprocess is repeated until all of the cores have been identified so thatthe locations of the ends of each core at the ends of the cable areknown. Any crossed or incorrectly wired cores may thus readily beidentified.

Various modifications may be made within the scope of the invention. Forinstance, the logic circuit 21 may be provided with manually operablecontrol means for permitting the various sequencing to be performedunder manual control ("one-shot" operation). A memory may be providedfor storing indications of faults together with indications of faultycores for subsequent analysis. The logic circuit 21 and thedecoder-driver 22 may be made from discrete logic or may include or beembodied by a microprocessor or the like.

    ______________________________________                                        Test Resistor Feedback Resistor                                               Value (Ohms)  Value (Ohms)                                                    ______________________________________                                        1650          825                                                             2K            1K                                                              2.37K         1.21K                                                           2.87K         1.43K                                                           3.4K          1.69K                                                           4.12K         2.05K                                                           4.99K         2.49K                                                           5.9K          3.01K                                                           7.15K         3.57K                                                           8.66K         4.32K                                                           10.5K         5.23K                                                           12.7K         6.34K                                                           15.4K         7.68K                                                           18.7K         9.31K                                                           22.6K         11.3K                                                           26.7K         13.3K                                                           31.6K         15.8K                                                           37.4K         18.7K                                                           44.2K         22.1K                                                           53.6K         26.7K                                                           64.9K         32.4K                                                           78.4K         39.2K                                                           95.3K         47.5K                                                           115K          57.6K                                                           187K          68.1K                                                           165K          82.5K                                                           191K          97.6K                                                           226K          113K                                                            267K          133K                                                            316K          158K                                                            374K          187K                                                            442K          221K                                                            523K          261K                                                            624K          316K                                                            768K          383K                                                            931K          464K                                                            562K + 562K   562K                                                            681K + 681K   681K                                                            825K + 825K   825K                                                            1 M + 1 M     1 M                                                             ______________________________________                                    

I claim:
 1. An apparatus for testing for leakage between cores of a multicore cable having a first end which is connected to said apparatus during testing and a second end which is disconnected during testing, said apparatus comprising:first means for connection to the first end of the multicore cable; second means for applying via said first means a test voltage to each of a plurality of first groups of the cores in turn for a predetermined period, each of the first groups of cores comprising at least one core; third means for connecting to a predetermined voltage via said first means each of a plurality of second groups of the cores in turn during an initial portion of the predetermined period and for disconnecting each of the second groups of cores from the predetermined voltage at the end of the initial portion of the predetermined period, each of the second groups of cores comprising at least one core; and fourth means for detecting current via said first means in each of the second groups of cores in turn during a portion of the predetermined period subsequent to the initial portion.
 2. An apparatus as claimed in claim 1, in which said second means for applying a test voltage comprises a test voltage source, and first switch means for connecting said at least one core of each of said first groups of cores in turn to the test voltage source.
 3. An apparatus as claimed in claim 1, in which said fourth means for detecting current comprises a current measuring meter, and second switch for connecting said at least one core of each of said second groups of cores in turn to the current measuring meter.
 4. An apparatus as claimed in claim 3, in which said third means comprises third switch means connected to said second switch means between the latter and said current measuring meter for connecting momentarily the output of said second switch means to the predetermined voltage after said second switch means connects each said at least one core of each of said second groups of cores to the current measuring means. 